As we saw at the beginning of the chapter it is important that we comprehend the nature of the covenant of works. Adam was given the possibility ex pacto to obtain the eschatological rest of the seventh day by his obedience. In a prefall world, therefore, Adam would be justified by his works. Some might object to using the term “justification” in connection with Adam, as there are inherently redemptive connotations associated with the term. If, however, to be justified is to be declared righteous, contrasted with condemnation as the declaration of a person’s wickedness, then Adam’s justification is based upon the completion of the agreed work of the covenant.
That Adam’s covenant work was to be marked by obedience sets an important element upon the table for consideration in one’s doctrine of justification. It means that an important aspect of the redemptive work of the last Adam must include obedience in his covenant work–fulfilling the failed work of Adam on behalf of the people of God. As G.C. Berkouwer notes, “The obedience of the crucified Christ—this is the alpha and omega of our justification. He covers our obedience with His obedience, our unrighteousness with His righteousness.” In technical terms, this means that an important element of Christ’s work as it concerns justification is not only his passive obedience, his obedience throughout his life connected with his passio and ultimately his crucifixion, but also his active obedience, his fufillment of the law. This brings up a second issue, namly the imputation of Christ’s obedience to those who look to him by faith.
That Adam forfeited the right to obtain the eschatological blessing by his obedience, requiring the work of the last Adam, explains why Paul excludes the works of the law as the ground for our justification: “Yet we know that a person is not justified by work of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified (Gal. 2:16). Now, while in this context Paul addresses the subject of the works of Torah, that is, those who sought to be justified on the basis of their obedience to the law of the Mosaic covenant, yet the principle of excluding works, or obedience, of any kind is rooted in Adam’s failure in the garden. In the world of Adam before the fall, one could be justified by works, but in the eschatological world of the last Adam, one can be justified only by faith.
One thing that is clear from Adam’s state in the garden-temple is that he did not represent himself alone-he was the federal head of mankind. Paul succinctly states that all men sinned because Adam sinned: “Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned” (Rom. 5:12). Paul’s statement is clear-the universal reign of death is due to the one sin of Adam. There is a parallel between the two federal heads, the first and last Adams: just as Adam’s disobedience and guilt are imputed to those whom he represents, so too Christ’s obedience, or righteousness, is imputed to those whom he represents. This means that one of the key elements of the doctrine of justification is the concept of imputation.
Taken from Justification: Understanding The Classic Reformed Doctrine by J.V. Fesko, pages 134-135.